1. Technical Field
This disclosure relates to devices that convert sound into operating signals and more specifically to microphones having multiple microphone arrangements.
2. Related Art
A coincident arrangement of gradient transducers may form a soundfield microphone (or B-format microphone). A soundfield microphone includes four pressure gradient capsules. The individual capsules are arranged in a tetrahedral form with the diaphragms of the individual capsules parallel to the imaginary surfaces of a tetrahedron. Each pressure gradient receivers delivers signals A, B, C or D and has a directional characteristic deviating from a sphere. The characteristic may be represented by k+(1−k)×cos(θ). θ denotes the azimuth, under which the capsule is exposed to sound. k indicates a percentage of omni signal (in a sphere, k=1, in a figure-of-eight, k=0). The signals of the individual capsules maybe denoted A, B, C and D. The axis of symmetry of directional characteristic of each individual microphone is perpendicular to the diaphragm and to the corresponding surface of the tetrahedron. The axes of symmetry of the directional characteristic of each individual capsule may form an angle of about 109.5° with each other.
According to one calculation, the four individual capsule signals maybe converted to a B-format (W, X, Y, Z):W=½(A+B+C+D)X=½(A+B−C−D)Y=½(−A+B+C−D)Z=½(−A+B−C+D)
The forming signals may form a sphere (W) and three figure-eights (X, Y, Z) that are orthogonal to each other. To configure the frequency and phase response of the directions, so that a flat energy characteristic is achieved with respect to the frequencies in the audible range, the signals W, X, Y, Z may be equalized. For a zero-order signal (W) and the first-order signals X, Y, Z, theoretical equalization characteristics depend on the frequency and effective distance of the center of the microphone capsules from the center of the tetrahedron.
The main directions of the figure eight X, Y, Z are normal to the sides of a cube enclosing the tetrahedron. By linear combination of at least two of these B-format signals, an arbitrary microphone capsule may be synthesized. Deviations from the theory based on the use of real capsules and the failure to satisfy ideally the coincidence requirement cause the performance of the synthesized microphones to deteriorate.
Synthesizing or modeling of the microphone may occur precisely in that the omni signal W is combined with one or more of the signals X, Y, Z, taking into account a linear weighting factor “r”. For directional characteristics in the area between a sphere and a cardioid, this may be derived for a synthesized capsule in the X-direction through the formula M=W+r×X, in which r can assume arbitrary values >0. The level of the signal M obtained may be normalized, so that the desired frequency trend is obtained for the main direction of the synthesized capsule. If a synthesized capsule is analyzed in any direction, additional weighting factors may be used, since rotation of the synthesized capsule in any direction may occur through a linear combination of three orthogonal figure-eights (X, Y, Z).
In a soundfield microphone the directional characteristic of the entire microphone may be adjusted. The microphone may be adapted even during playback or a final production of a sound carrier. It is possible to focus on a corresponding soloist of an ensemble, to mask out unexpected and undesired sound events by influencing the directional characteristic, or to follow a moving sound source (for example, a performer on the stage), so that the recording quality remains independent of the changed position of the sound source.
When sound is recorded from a soundfield microphone, the entire sound field may be described at any location in time. Time differences, etc., may be analyzed during selected evaluations. When deviations occur, the coincidence conditions for small wavelengths may no longer be satisfied. Distortions and artifacts may occur with respect to the frequency response and directional characteristic of a synthesized signal. A rotation of each individual gradient capsule of the soundfield microphone of about 180°, so that each of the four diaphragm surfaces is brought closer to the center, has shown that artifacts may not be eliminated at higher frequencies. Acoustic shadowing of the front microphone mouthpieces may not alter the limit frequency, up to which the calculation method applies.
There is a trade-off between the coincidence requirement and the attainable noise distance of the employed gradient capsules. The larger the individual diaphragm surface, the more noise distance may be achieved. However, this relationship leads to a larger distance of the diaphragm surfaces to the center of the arrangement. An optimal solution requires positioning of the four individual capsules as closely as possible to each other, so that the sound inlet on the back of the gradient transducer is influenced by the resulting structure of the closely positioned capsules. This means that the cavity formed in the interior of the microphone arrangement, and naturally also its delimitation by the microphone arrangement, as well as its mounts, etc., will act as an acoustic filter. The acoustic filter may affect the acoustic filtering by the sound paths that lead to the back of the individual capsules. The effect of this additional acoustic filter is frequency-dependent and may have its strongest effect at frequencies at which the wavelength of the sound is about the same order as the dimensions of the diaphragm or the dimensions of the entire soundfield microphone. In some soundfield microphones, this effect may occur in the frequency range around 10 kHz, at which rejection, (e.g., the frequency response from the direction from which the individual capsule is least sensitive becomes weakest and, drops below 10 dB).
In some soundfield microphones, two of the capsules may be situated with their main direction positioned downward, which means that they may be particularly sensitive to non-ideal microphone mounting or fastening under practical conditions. Such acoustic disturbances, based on the capsule arrangement, may develop due to reflections on the mounting material, on the floor, etc. In addition, the capsules in the close arrangement may be influenced when the theoretically rotationally symmetric directional characteristic of the synthesized omni signal is disturbed.
In some soundfield microphones a common configuration (X-Y-plane) is achieved by switching four capsule signals. The B-format signals in the X-Y-plane may be formed from microphone signals that meet at an angle of about 54° in each capsule under the influence of sound. If a directional diagram of a gradient transducer is considered, scattering of the rejection angle of the individual capsules may have a stronger effect, as the inlet direction deviates from the main direction. If two capsules with slightly different polar patterns exposed to sound from 0° differ only by the sensitivity described, at angles greater than 0°, the difference is increased by a percentage as a result of the difference in rejection angles.